1. Field of the Invention
The present invention relates to a system and a method of concentrating a spent caustic stream at a chemical facility. More particularly, the present invention relates to the removal of water and volatile organic compounds including mercaptans from a spent caustic stream at a refinery.
2. Description of Prior Art
A caustic stream at a chemical facility such as a refinery may be a 3-15% solution of sodium hydroxide in water. Caustic streams are used to treat other streams to remove unwanted compounds such as chloride, fluoride and hydrogen sulfide from gasoline or LPG streams. When the concentration of the sodium hydroxide decreases to about 1%, the stream is treated as a spent stream. A spent caustic stream is disposed of because of the large quantity of compounds, e.g. other sodium compounds, it has acquired while treating other streams. One method of disposal is to feed the spent caustic stream to an incinerator.
It is known to concentrate a spent caustic stream at a refinery by feeding the stream to a flash drum, wherein vaporized water is removed from the overhead of the flash drum and condensed for further treating in a sewer treatment plant. Concentrated spent caustic is then pumped from the flash drum to a spent caustic incinerator. Such concentration scheme decreases the throughput of the incinerator. However, the vaporization mechanism in the flash drum also boils off volatile organic compounds (VOC's) including trace quantities of mercaptans. These VOC's entrained with the water sent to the sewer treatment plant can generate a very strong objectionable odor of mercaptans in the sewer plant.
Typically, the flash drum effluent of the concentrated spent caustic may be at a temperature of about 100.degree. C. This hot concentrated caustic stream also contains hydrocarbons, such as gasoline and kerosine, which raises the possibility of ignition of the hydrocarbons and resultant heating of the piping on the way to the caustic incinerator.